Add: progressive morphological filter to extract ground returns
Add: filter to remove locally maximal points
New: trajkovic and hedley detector 2D
New: port of Trajkovic and Hedley detector to pure geometry
Fix: a silly bug in normals diff computation
Add: pcl_ni_trajkovic application to detect Trajkovic 2D/3D points from OpenNi device
Fix: removal of unused argument and normals_->resize statement
Add savePNGFile function for histograms.

Render the histogram using vtk offline and then save the image to png
file.
PCDGrabberImpl: Add mutex to avoid reentry in trigger()/readAhead() and the resulting race condition.
Add CentroidPoint class
Add computeCentroid() functions that use CentroidPoint
Extract centroid-related tests from test_common.cpp

There are quite a number of functions dealing with centroid, mean, and
covariance computation. They all are grouped in 'common/centroid.h'
header. Therefore it makes sense to group their corresponding test
functions in a separate file. Hence creation of 'test_centroid.cpp'.
Add unit tests for CentroidPoint and computeCentroid()
Fix obscure MSVC error
Change all "%zu" in *.h *.hpp and *.cpp to "%lu".
Fix "unused-parameter" warning in 'accumulators.hpp'